Synchronous Systolic Subcellular Ca ²⁺ -Elevations Underlie Ventricular Arrhythmia in Drug-Induced Long QT Type 2

Abstract

Background Repolarization-delay is a common clinical problem which can promote ventricular arrhythmias. In myocytes, abnormal sarcoplasmic reticulum Ca2+-release is proposed as the mechanism that causes early afterdepolarizations, the cellular equivalent of ectopic-activity in drug-induced long QT syndrome. A crucial missing link is how such a stochastic process can overcome the source-sink mismatch to depolarize sufficient ventricular tissue to initiate arrhythmias. Methods and Results Optical maps of action potentials (APs) and Ca2+-transients (CaT) from Langendorff rabbit hearts were measured at low (150×150 μm2/pixel) and high (1.5×1.5 μm2/pixel) resolution before and during arrhythmias. Drug-induced long QT type 2, elicited with dofetilide inhibition, produced spontaneous Ca2+-elevations during diastole and systole, before the onset of arrhythmias. Diastolic Ca2+− waves appeared randomly, propagated within individual myocytes, were out-of-phase with adjacent myocytes and often died-out. Systolic secondary Ca2+− elevations were synchronous within individual myocytes, appeared 188±30ms after the AP-upstroke, occurred during high cytosolic-Ca2+ (40–60% of peak-CaT), appeared first in small islands (0.5×0.5 mm2) that enlarged and spread throughout the epicardium. Synchronous systolic Ca2+-elevations preceded voltage-depolarizations (9.2±5ms, n=5) and produced pronounced Spatial Heterogeneities of CaT-durations and AP-durations. Early afterdepolarizations originating from sites with the steepest gradients of membrane-potential propagated and initiated arrhythmias. Interestingly, more complex subcellular Ca2+-dynamics (multiple chaotic Ca2+-waves) occurred during arrhythmias. K201, a ryanodine receptor stabilizer, eliminated Ca2+-elevations and arrhythmias. Conclusions The results indicate that systolic and diastolic Ca2+-elevations emanate from sarcoplasmic reticulum Ca2+-release and systolic Ca2+-elevations are synchronous because of high cytosolic and luminal-sarcoplasmic reticulum Ca2+ which overcomes source-sink mismatch to trigger arrhythmias in intact hearts.

The link between abnormal calcium handling and electrical instability in acquired long QT syndrome – Does calcium precipitate arrhythmic storms?

“…Treatment with K201 (1 μM to avoid off-target effects), a RyR stabilizer, had a similar effect. 42 Taken together, these results provide compelling evidence that in this animal model, spontaneous systolic SR Ca 2+ release, manifested as a SSCE, cause EADs. The observation that L-type Ca 2+ channel blocker and low extracellular Ca 2+ also prevent EAD appearance is not surprising, since both interventions are expected to deplete SR Ca 2+ load.…”

“…42 This is largely absent at baseline, when spatial the amplitude of both CaT and AP is relatively even across the epicardial surface at all times, but increases dramatically with the appearance of Ca oscillations. Intriguingly, the areas of high calcium oscillation amplitude are highly irregular, do not appear to follow obvious anatomic regions and differ among different experiments, although they generally remain relatively stable between subsequent beats.…”

Evaluation of beat-to-beat ventricular repolarization lability from standard 12‐lead ECG during acute myocardial ischemia

“…Non-alternans repolarization variability has been initially reported in congenital long QT syndrome [20] and it is also related to abnormal myocardial calcium handling in animal experiments [21–23]. It is uncertain if NARV is a prominent feature of acute myocardial ischemia, though it seems to increase prior to onset of ventricular arrhythmias in patients with coronary artery disease [9, 24].…”

K201 (JTV519) is a Ca2+-Dependent Blocker of SERCA and a Partial Agonist of Ryanodine Receptors in Striated Muscle

“…In principle, these differences with our results may relate to the lack of ATP/Mg 21 and the very low luminal Ca 21 in the SR in [ 3 H] ryanodine binding studies, which made RyRs less sensitive to agonists (Sitsapesan and Williams, 1990;Ogawa, 1994;Fill and Copello, 2002 A SERCA Role for the Cell-Protective Action of K201? There is consensus that the "normalization" of EC-coupling induced by K201 correlates with improved cardiovascular cell function in pathologies, including arrhythmia and heart failure (Yano et al, 2003;Wehrens et al, 2005;Loughrey et al, 2007;Toischer et al, 2010;Driessen et al, 2014;Sedej et al, 2014;Kim et al, 2015). K201 also benefits ischemic SkM (Wehrens et al, 2005).…”